The application of a texture to a surface implies a modification to the surface structure. An example in the semiconductor industry is the planarization of sputtered aluminum by laser heating as described in the article entitled "Laser Planarized Metal Shows Several Advantages" by P. H. Singer, chief editor, Semiconductor International. vol. 13, No. 6, May 1990, pages 18 and 20. The heating process causes the aluminum to melt and flow into vias thereby forming a smooth planar surface. The smoothed surface allows for simpler processing steps for subsequent fabrication of multilayered semiconductor devices. The application of a roughened surface also has a variety of applications. In what can be called "passive" applications, a roughened surface can be used to mark or label a surface. In this case the roughened surface may also be in such a pattern as to be decorative in form. In so called "active" applications, the rough surface can be used, for example, to inhibit reflections or increase coefficients of friction.
One particular example for the need to inhibit reflections is found in the fabrication of backside illuminated charge coupled devices (CCDs). CCDs are solid state electronic imaging devices which read out image charges from wells in an array of pixels. CCDs designed for solid-state cameras, such as camcorders, are in great demand and are widely available. They have been designed to provide adequate performance when viewing brightly illuminated scenes. However, in astronomical, scientific and military applications their spectral response, readout noise, dark current, full well-capacity and blooming characteristics are not satisfactory.
To overcome the limitations of imaging through the polysilicon gates that necessarily cover all of the sensitive pixel array, it would be desirable to illuminate the CCD from the backside if the silicon substrate were thin enough. In other words, a solution to obtaining better light sensitivity would be the thinning of the backside of the CCD to a total thickness of roughly 10 microns. The need is quite apparent for new microelectronic processing schemes to produce thin membranes such as those required for the backside illuminated CCDs, such a process is disclosed in the above identified S. D. Russell et al. "Excimer Laser-Assisted Etching of Silicon Using Halocarbon Ambients", U.S. patent pending application Ser. No. 07/501,707. Additional features that should attend this thinning process are the creation of a smooth surface for uniform imaging, nonreflecting sidewalls for stray light rejection and large (approximately 2 mm by 2 mm) square cross section for optimal illumination of the active area of the array.
A conventional fabrication procedure for backside illuminated CCDs calls for chemical thinning of the silicon. However, the standard wet chemical thinning-etch procedure produces an extremely low yield process and requires the handling of fragile thin silicon membranes. Furthermore, the chemical thinning requires two processes, a deep etch using potassium hydroxide and a subsequent Dash polishing etch. The active area of the array is normally fabricated by chemically thinning a (100) oriented silicon wafer using the potassium hydroxide (KOH) etch. The potassium hydroxide anisotropically etches to the (111) crystallographic plane in silicon leaving smooth sidewalls at an angle of 54.7 degrees to the surface of the array (as described in "Micromachining of Silicon Mechanical Structures" by G. Kaminsky in J. Vac. Sci. Technol. B. Vol. 3, No. 4, July/August 1985, pages 1015-1024) and shown in FIG. 1. This smooth surface acts as a mirror to reflect extraneous light onto the active array of the CCD causing spurious images thereby degrading response uniformity, image resolution and increasing background (dark) signal. Under stress conditions such as a radiation environment, these parameters degrade further subsequently failing to meet device specifications leading the problems mentioned above to be exascerbated. Therefore, there is a need in the fabrication of CCDs to roughen these sidewalls without causing damage to any other portion of the electronic circuit. The Dash etch mentioned above, consists of applying a mixture of acetic, nitric and hydrofluoric acids along with a surfactant. The Dash etch process used for polishing the thinned membrane also requires additional masking to protect the frontside metalization and backside gold eutectic used for packaging and therefore does not eliminate sidewall reflections. Additional cleaning and inspection steps are required to complete the conventional thinning process. Elimination of these steps would allow further "dry" processing of the thinned die, such as laser doping or dopant activation. In addition, the minimization of the required number of processing steps always is desirable in this microelectronic processing procedure to maximize the yield and reliability while also reducing costs.
The preferred method proposed here utilizes an excimer laser in a non-contact process to promote a chemical reaction between a halocarbon ambient and the sample. The laser-assisted chemical reaction results in a roughened surface which will not scatter light. The use of a non-reactive ambient allows for texturing prepackaged and pretested devices thereby minimizing fabrication costs and salvaging devices which fail specifications. Subsequent laser processing steps, such as activation of backside implant, can be easily implemented before or after this process.
It is evident from the above discussion that advances in microelectronics often are limited by the multitude of relatively complicated processing steps required to produce the devices. A further example of the large number of processing steps required in a typical process flow is shown by a simple etching of a pattern into bulk silicon. This requires the application of a precise thickness and uniform film of photoresist with a subsequent low temperature heat treatment. This is followed by exposure to a lamp through a mask in contact with the photoresist and chemical development of the resist. Another heat treating step is next, then the silicon is chemically etched and, lastly, the remaining photoresist subsequently is stripped from the silicon. These seven steps are typical in standard etching techniques used in the semiconductor industry. It becomes apparent that significant savings and yield could be obtained through the more simplified procedures that might be provided by a contactless form of etching. The reduced complexity of such a procedure would eliminate the many time consuming and costly steps of the conventional etching technique, and in the case of roughening the sidewalls of a backside illuminated CCD provide a unique method to rework tested parts.
In view of the foregoing, noncontact processing is receiving widespread interest in the microelectronic industry. A variety of laser-assisted processing techniques to modify materials used in this industry are being pursued, particularly with the introduction of the excimer laser which typically emits at the shorter wavelengths. The works of D. Ehrlich et al. in their article "A review of Laser-Microchemical Processing" J. Vac. Sci. Technol. B., 1, 969 (1983), F. Houle, in her article entitled "Basic Mechanisms in Laser Etching and Deposition" Appl. Phys. A. 41, 315 (1986), D. Bauerle in the article entitled "Chemical Processing with Lasers: Recent Developments" Appl. Phys. B, 46, 261 (1988), and T. Chuang in the article entitled "Laser-Induced Chemical Etching of Solids: Promises and Challenges" in A. Johnson et al., ed's, Laser Controlled Chemical Processing of Surfaces, Materials Research Society Symposia Proceedings, vol. 29 (New York: North Holland, 1984), pp. 185-194, offer a review of the effort involved with laser-assisted processing techniques. As a consequence, laser processing has grown from a purely research effort into a production tool. Early on, however, investigations related to laser processing of silicon led to the conclusion that laser ablation of silicon using an excimer laser was considered undesirable since the surface quality would be poor, although the rate of material removal would be high. For texturing applications where particulate control is of little or no concern, ablative techniques are applicable. However, the use of ablation for texturing applications has not been previously reported.
The use of halogens to etch silicon is well established by the text of the Gutmann, Halogen Chemistry. Vol 2 (New York: Academic Press, 1967), pp. 173-180. In addition, an existing body of research for plasma processing of silicon is described by H. F. Winters et al. in the article "Surface Processes in Plasma-Assisted Etching Environments" J. Vac. Sci. Technol. B, 1, 469 (1983) and B. A. Heath et al. in the article "Plasma Processing for VLSI" chapter 27 in M. G. Einspruch, ed. VLSI Handbook (San Diego: Academic Press, 1985) pp. 487-502.
The laser-assisted etching of silicon has been examined using a chlorine ambient by R. Kullmer et al. in their article "Laser-Induced Chemical Etching of Silicon in Chlorine Atmosphere: I. Pulsed Irradiation" Appl. Phys. A, 43, 227 (1987), P. Mogyorosi et al. in the article entitled "Laser-Induced Chemical Etching of Silicon in Chlorine Atmosphere: II. Continuous Irradiation" Appl. Phys. A, 45, 293 (1988), R. Kullmer et al. in the article "Laser-Induced Chemical Etching of Silicon in Chlorine Atmosphere: Combined CW and Pulsed Irradiation" Appl. Phys. A, 47, 377 (1988), Y. Horiike et al. in the article "Excimer Laser Etching on Silicon" Appl. Phys. A, 44, 313 (1987) and W. Sesselmann et al. in their article entitled "Chlorine Surface Interaction and Laser-Induced Surface Etching Reactions" J. Vac. Sci. Technol. B, 3, 1507 (1985). S. Palmer et al. in their article entitled "Laser-Induced Etching of Silicon at 248 nm in F.sub.2 /Ne" Conference on Lasers and Eleectrooptics Technical Digest Series 1988. vol. 7, 284 (Optical Society of America, Washington, D.C., 1988) examined the fluorine ambient.
The use of nitrogen trifluoride ambient was discussed by J. H. Brannon in his article entitled "Chemical Etching of Silicon by CO.sub.2 Laser-Induced Dissociation of NF.sub.3 " Appl. Phys. A, 46, 39 (1988) and the above referenced article by Y. Horiike et al. The use of the halogenated ambient xenon difluoride was discussed by T. Chuang, "Infrared Laser Radiation Effects on XeF.sub.2 Interaction with Silicon" J. Chem. Phys., 74, 1461 (1981), by F. Houle "Photoeffects on the Fluorination of Silicon: I. Influence on Doping on Steady State Phenomena" J. Chem. Phys., 79, 4237 (1983) by F. Houle in the article "Photoeffects on the Fluorination of Silicon: II. Kinetics of the Initial Response to Light" J. Chem. Phys., 80, 4851 (1984). And, the use of halogenated ambient sulphur hexafluoride was examined by T. Chuang in his article entitled "Multiple Photon Excited SF.sub.6 Interaction with Silicon Surfaces" J. Chem. Phys., 74, 1453 (1981).
Typical etch rates of approximately one angstrom per pulse have been reported for the ambients of the preceding paragraphs under a variety of conditions. Such etch rates with the high pulse repetition rate of the excimer laser (100 Hz typical, 250 Hz available) are satisfactory to meet yield requirements of some applications. However, difficulties in handling and processing pure halogens such as chlorine and fluorine make them less suitable for inserting into existing manufacturing processes. Furthermore, pure halogens and many halogenated ambients are corrosive in nature and will spontaneously react with some (or all) of the materials composing a partially fabricated microelelectronic device. Masking, therefore, is required in such ambients despite the non-contact nature inherent in laser processing. Masking may be difficult or impossible in many applications. In addition, the detrimental effects of chlorine on the radiation hardness of silicon devices makes it potentially unsuitable for a wide variety of military or space applications.
The use of laser-assisted wet etching was explored by R. Osgood Jr. et al. in "Localized Laser Etching of Compound Semiconductors in Aqueous Solutions" Appl. Phys. Lett., 40, 391 (1982), R. von Gutfeld et al. in "Laser-Enhanced Etching in KOH" Appl. Phys. Lett., 40, 352 (1982) and F. Bunkin et al. in "Laser Control Over Electrochemical Processes" SPIE Vol. 473, Symposium OPPIKA' 84, Vol. 473, pp. 31-37. The drawback to the laser-assisted wet etching technique is that it requires a different processing chamber to that of the gaseous "dry" etching technique and would require additional handling for further processing.
M. D. Armacost, S. V. Babu, S. V. Nguyen, J. F. Rembetski, in their article "193 nm Excimer Laser Assisted Etching of Polysilicon", Mat. Res. Soc. Symp. Proc. Vol. 76, (1987), pp. 147-156, examine various ambients for etching polysilicon. They used two halocarbon ambients but found etch profiles that were not repeatable or did not show any appreciable etching. They did not examine the effects of etching silicon, nor investigate the critical parameters and processing windows required to achieve the results attained in accordance with this inventive concept. S. D. Russell and D. A. Sexton in our article entitled "Excimer Laser-Assisted Etching of Silicon using Chloropentafluoroethane" in R. Rosenberg, A. F. Bernhardt, J. G. Black, eds. In-Situ Patterning: Selective Area Deposition and Etching, (Mat. Res. Soc. Proc., Vol. 158, Pittsburgh, Pa., 1990), pages 325-330, we discuss one possible laser-assisted chemical reaction used in laser texturing, however there is no mention of the resulting surface texture nor the effect and benefits of texture as disclosed here.
Thus, there is a continuing need in the state of the art for a contactless technique utilizing an excimer laser to promote a chemical reaction between an ambient and a sample thereby imparting a surface texture that produces the above mentioned benefits, eliminates many standard processing steps and has the advantage of processing in a prepackaged and pretested device and that can be extended to other applications requiring surface modification techniques.